.WAHPETON, N.D. (AP) – Residents were allowed to return to their homes after an early morning fire at an industrial plant here forced the mandatory evacuation of about 500 people.

The evacuation order was lifted Monday afternoon after hazardous materials teams from Fargo and Morris, Minn., and state Health Department officials tested the air and water quality, Richland County Emergency Manager Brett Lambrecht said.

“People are being allowed back in, but we’re asking them to shelter in place,” Lambrecht said. “We want them to stay in their homes to avoid extra traffic in the area.”

Lambrecht said residents in about a 40-block area were ordered to evacuate as a precaution for chemically tainted smoke. Two temporary shelters were set up in Wahpeton, but most people stayed with friends or relatives until they were allowed to return, he said.

One firefighter from nearby Dwight was treated for minor chemical burns to his feet, but later returned to the scene, said Dick Dickerson, a Dwight firefighter. Nine other firefighters were checked for possible chemical exposure and released, Lambrecht said.

No residents required treatment, Lambrecht said.

The Industrial Plating Corp. plant uses hydrochloric acid, sodium hydroxide and zinc for cleaning metal. Some of the runoff reached the Red River before the storm sewer system was closed, but it was not believed to be a hazard because the chemicals were diluted, Lambrecht said.

Firefighters doused the building with foam after the fire burned itself out, Lambrecht said.

Gov. John Hoeven toured the scene Monday afternoon, along with Maj. Gen. David Spryczynatyk, the National Guard commander, and Greg Wilz, state emergency services director. Several state agencies were helping containment and testing, Hoeven said.

The plant employs about 50 people, Hoeven said.

“Our immediate concern is safety,” Hoeven said. “But we’re also concerned about the employees.”

The cause of the fire was not immediately known. The state fire marshal was expected to inspect the plant on Tuesday, Lambrecht said.

Klovstad said the plant had an electrical fire two weeks ago. “We were hoping it was something that minor again,” he said.

About 100 firefighters from Wahpeton, Dwight and Breckenridge, Minn., responded to the blaze, Lambrecht said. Dickerson said he got the call at 2:30 a.m.

“We set up our attack line and started squirting water on it, but there wasn’t much we could do,” said Dickerson, standing a few feet away from a puddle of green water that firefighters described as pea soup. Officials constructed a ring dike to contain some of the water.

“I think that’s the stuff we’re staying out of,” Dickerson said, pointing to the puddle.

Authorities said the fire was reported early Monday morning by a police officer on patrol, and flames were soon leaping 20 feet into the air.

A handful of firefighters fell into chemicals after one of the nozzles “let loose on them,” Klovstad said.

“They all came back here and got decontaminated,” Klovstad said from the Wahpeton fire station. “They all got a change of clothes and I sent them to the hospital to make sure they’re OK.”

Officials said bitter cold temperatures slowed the firefighting effort.

“I dread any fire, whether it’s cold or warm,” Klovstad said.